Previous studies suggest that LH and progesterone (P) are involved in the regulation of the monkey luteal structure and function during the menstrual cycle. However, the role of gonadotropin and P in the maintenance of monkey luteal structure during early pregnancy remains unknown. To simulate early pregnancy (SEP), rhesus monkeys received increasing doses of human CG (hCG) beginning on day 9 of the luteal phase. hCG was given alone or with the 3b-hydroxysteroid dehydrogenase inhibitor trilostane (500 mg/dose, BID) to reduce serum P. Corpora lutea were removed 1, 3, 6, and 9 days (n=3/group) after initiation of hCG + trilostane treatment, weighed, and prepared for histologic examination. Corpora lutea were also obtained from time-matched, untreated (control) monkeys. Daily serum samples were assayed for P and estradiol (E) levels by RIA. Treatment with hCG maintained elevated serum P throughout the treatment interval, while administration of hCG + trilostane reduced P to very low levels the day after initial treatment and maintained these low levels throughout the study period. E was reduced, but not ablated, in hCG + trilostane-treated animals when compared with those receiving hCG only. Luteal weight decreased in control monkeys over the study interval. hCG treatment maintained luteal weight through day 9, while hCG + trilostane treatment maintained luteal weight only through day 6. By day 9, the weight of luteal tissue from hCG + trilostane treated animals was similar to controls. In control animals, luteal tissue was healthy in appearance on days 1 and 3 but showed signs of structural luteolysis such as shrinking luteal cell cytoplasm and pyknotic nuclei by the end of the study interval. hCG treatment maintained luteal structure thoughout the study period. Treatment with hCG+trilostane maintained luteal structure through day 6. By day 9, reduced luteal cell size and nuclear degradation were evident. hCG + trilostane administration also stimulated follicular maturation as early as day 3, with multiple follicles as large as 5 mm observed by day 9. While treatment with hCG alone maintained luteal structure beyond the end of the normal luteal phase, structural luteolysis was evident following hCG + trilostane administration, supporting a role for P or a metabolite in the maintenance of luteal structure.